The invention relates to a method for monitoring the state of a safety bearing of a machine. The invention also relates to a relevant machine.
To support the rotating rotor shaft in a machine magnetic bearings, which hold the rotating rotor shaft in a floating state, are being used ever more frequently to support the rotor shaft during operation. If the magnetic bearing fails, as a result of a power failure for example, the rotor shaft falls into a safety bearing and is caught by the latter. The safety bearing thus serves to catch the rotor shaft. The safety bearing temporarily takes over the support of the rotor shaft until the rotor shaft has come to a complete standstill. Safety bearings must on the one hand resist the impact when the rotating shaft drops down into the safety bearing and on the other hand guarantee that the rotor shaft coasts safely to a halt. For this purpose the bearing ring of the safety bearing has a slightly larger internal diameter compared to the rotor shaft diameter, so that in normal operation, i.e. when the magnetic bearing is active, the rotor shaft does not touch the safety bearing. Usually the safety bearing is accommodated in the stator housing in the area of the respective end of the rotor shaft.
When catching the rotor shaft the safety bearing will be subjected to considerable stresses which lead to wear on the safety bearing. The life of the safety bearing is shortened by the wear with, in the worst case the safety bearing, because of the heavy wear which can even occur during a single runout of the rotor shaft into the safety bearing, no longer being capable of being used for a further crash even after a single crash of the shaft into the safety bearing. The safety bearings are built into the machine and as a rule cannot be inspected there without dismantling parts of the machine. The optimum moment for replacing a defective or deteriorating safety bearing thus cannot be established with any certainty. The problem has been solved previously by counting the crashes. If a predefined number of crashes of the rotor shaft into the safety bearing is exceeded, e.g. for five crashes, the safety bearing must be replaced. The safety bearing can however already be worn out after fewer than five crashes or also stand up to significantly more crashes. The first case can result in failure of the safety bearing, the second case in unnecessary and expensive downtimes of the machine in order to replace safety bearings that are not yet defective or worn.